Four of us visited DC last August, splitting into two couples for the dolphin encounter so that we could film each other from the 'shoreline'. We were very pleased with the results, often much better than the official videographer who comes into the water with you. If you want to do this it is worth spending a few minutes to scout out good locations around the cabanna (one of 6[?] small coves where the dolphins swim) from which to film. You will not be allowed to take any kind of camera into the dolphin swim.
The rest of the park is camera friendly. I have a waterproof case for my videocamera and got some great shots of all of us swimming in the lazy river and the coral reef area.
Regards
Rob